Jetpack: site_inaccessible, HTTP 500 error

I get the error message, ‘Your website needs to be publicly accessible to use Jetpack: site_inaccessible / Error Details: The Jetpack server was unable to communicate with your site [HTTP 500]. Ask your web host if they allow connections from WordPress.com. If you need further assistance, contact Jetpack Support: http://jetpack.me/support/’.

I have quite a few WordPress installations on the same server but different domains, and they all work fine. AFAIK I have used exactly the same procedure setting this one up as the others.

The xmlrpc.php file is present but when I call it from my web browser, I get a blank page. I do see the site so I don’t believe it’s a DNS issue. I registered the domain last Friday and installed WordPress on Saturday.

I have no plug-ins other than Akismet, which I have put my API key in to (the blog says it is valid): this is a fresh installation.

Hope you can point me in the right direction! I have read numerous pages here already but nothing appears to fit my circumstances.

Kind regards,

Jack

PS.: The WordPress Support form asks me to enter my Blog URL and the blog I am writing about does not appear below. I can’t figure out how to add it to the list, so I’ve chosen one of my others for now.
Blog url: http://jackyan.com/